43일차 - Convert PascalCase string into snake_case

--------------------------------------------🤞 My Solution -------------------------------------------

def to_underscore(string):
    arr = []
    trans = []
    string = str(string)
    for i in range(0, string.__len__()):
        arr.append(string[i])
    arr[0] = arr[0].lower()
    trans.append(arr[0])
    for i in range(1, string.__len__()):
        if arr[i].isupper():
            arr[i] = arr[i].lower()
            trans.append('_')
            trans.append(arr[i])
        else:
            trans.append(arr[i])
    result = ''
    for i in range(0, trans.__len__()):
        result += trans[i]
    return result